The present invention relates to a gastight, sealed metal oxide/metal hydride storage battery having a cell casing for containing a positive electrode comprised of a metal oxide, a negative electrode comprised of a hydrogen storage alloy, a separator disposed between the positive and negative electrodes which contains an alkaline electrolyte, and an auxiliary electrode for maintaining oxygen consumption which is in electrically conducting contact with the negative electrode.
A preferred field of application of the present invention is in button cells of the Ni/metal hydride type. This is because of their efficiency and their established use in mains-independent devices. However, the load-carrying capacity of such cells for high currents generally does not reach that of cylindrical cells. This is believed to be due to the conductivity of the negative electrode of a button cell, which is relatively low. This lower conductivity is due to the proportion of the plastic binder in the mass mixture, which is preferably composed of PTFE and which can amount to 10% of the mass. However, the addition of this binder is necessary for reasons of mechanical strength, and to impart a hydrophobic nature to the mass to create the conditions needed for adequate gas consumption in the event of overcharging and polarity reversal.
Rechargeable, gastight metal oxide/metal hydride button cells, as a generic class, must be both capable of overcharging and protected against polarity reversal. During overcharging, oxygen is generated at the positive electrode and has to be consumed to avoid an unacceptably high internal pressure. A means suitable for this purpose is the auxiliary electrode, which is usually composed of a mixture of carbon black, graphite and PTFE. The auxiliary electrode is generally situated to the rear of, and is electrically short-circuited to the negative (main) electrode. The mechanism by which oxygen consumption occurs can be described by the equation: O.sub.2 +4e.sup.- +4H.sub.2 O=4 OH.sup.- (Equation 1). An example of a metal oxide/metal hydride button cell having an auxiliary electrode for such oxygen consumption is described in German Offenlegungsschrift 39 29 306 (with reference to FIG. 4).
If a metal oxide/metal hydride storage battery enters polarity reversal (after exhaustive discharge of the positive electrode, which generally limits the discharge), the positive electrode will then develop hydrogen. This hydrogen can be absorbed directly by the negative electrode (with a conversion to metal hydride). However, this can take place only at a metallic surface which is in direct communication with the gas phase of the battery system. A liquid film at the metallic surface will considerably impede hydrogen absorption. The presence of a hydrophobic binder can keep the electrolyte away from the electrode surface and thus facilitate the penetration of hydrogen into the interior of the negative mass.
German Offenlegungsschrift 40 29 503 also discloses an alkaline storage battery having a negative metal hydride electrode. The active mass of the disclosed electrode further includes a special catalyst composed of Raney nickel or Raney silver, which are specifically added for the oxidation of hydrogen which is produced in the event of a polarity reversal.
Nevertheless, there remains the disadvantage of reduced electrical conductivity due to the use of a hydrophobic binder. For mass electrodes, while the binder acts to ensure the cohesion and hydrophobicity of the compacted powder mass, it also acts as an electrical insulator. As a result, good protection against polarity reversal and good current-carrying capacity tend to be mutually exclusive.